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            <h1 class="post-title">设计模式之设计原则-迪米特法则 (LoD)</h1>
            
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                    <i class="iconfont icon-calendar"></i> 发表于 2020-02-06
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                                <a href="/categories/%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F/"> 设计模式</a>
                            
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                                <a href="/categories/%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F/"># 设计模式</a>
                            
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            <p><a href="https://emojipedia.org/backhand-index-pointing-right/" target="_blank" rel="noopener">👉</a><a href="https://github.com/zhaohaihao/java-design-patterns/tree/master/005-LoD" target="_blank" rel="noopener"><strong>文章示例代码</strong></a><a href="https://emojipedia.org/backhand-index-pointing-left/" target="_blank" rel="noopener">👈</a></p>
<p>迪米特法则也叫做最少知道原则。</p>
<h1 id="定义"><a href="#定义" class="headerlink" title="定义"></a>定义</h1><blockquote>
<p>一个对象应该对其他对象保持最少的了解。</p>
</blockquote>
<p>在《Head First 设计模式》一书中，针对该原则也强调：只和你的密友交流。</p>
<p>在程序中，出现在成员变量、方法的输入、输出参数中的类我们可以称之为成员朋友类，而出现在方法体内部的类不属于朋友类。</p>
<p>这些个成员朋友类的对象和当前对象之间存在关联、聚合或者组合关系，可以直接访问这些成员朋友类对象的方法。</p>
<h1 id="场景示例"><a href="#场景示例" class="headerlink" title="场景示例"></a>场景示例</h1><p>假设现在超市老板来超市向职工说要看看目前超市内的商品总数量。</p>
<p>这里会牵扯到几个实体类：老板类、职工类、商品类。</p>
<h2 id="创建商品类"><a href="#创建商品类" class="headerlink" title="创建商品类"></a>创建商品类</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 商品类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-07 11:00</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Goods</span> </span>&#123;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="创建职工类"><a href="#创建职工类" class="headerlink" title="创建职工类"></a>创建职工类</h2><p>在职工类的内部实现一个职工查询商品数量的方法。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 职工类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-07 11:02</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Staff</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 职工查询商品数量</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> goods 商品列表</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">checkNumber</span><span class="params">(List&lt;Goods&gt; goods)</span> </span>&#123;</span><br><span class="line">        System.out.println(<span class="string">"目前超市内商品总数为: "</span> + goods.size());</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="创建老板类"><a href="#创建老板类" class="headerlink" title="创建老板类"></a>创建老板类</h2><p>在老板类的内部实现老板要求职工查询商品数量的方法。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 老板类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-07 10:59</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Boss</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 老板要求职工查商品的数量</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> staff 职工</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">requireCheckNumber</span><span class="params">(Staff staff)</span> </span>&#123;</span><br><span class="line">        List&lt;Goods&gt; goodsList = <span class="keyword">new</span> ArrayList&lt;Goods&gt;();</span><br><span class="line"></span><br><span class="line">        <span class="comment">// 这里模拟从数据库查商品数量</span></span><br><span class="line">        <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">50</span>; i++) &#123;</span><br><span class="line">            goodsList.add(<span class="keyword">new</span> Goods());</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        staff.checkNumber(goodsList);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="测试类及输出"><a href="#测试类及输出" class="headerlink" title="测试类及输出"></a>测试类及输出</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 测试类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-07 11:09</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        Boss boss = <span class="keyword">new</span> Boss();</span><br><span class="line">        Staff staff = <span class="keyword">new</span> Staff();</span><br><span class="line"></span><br><span class="line">        boss.requireCheckNumber(staff);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>测试类的输出结果如下：</p>
<blockquote>
<p>目前超市内商品总数为: 50</p>
</blockquote>
<h2 id="类结构图"><a href="#类结构图" class="headerlink" title="类结构图"></a>类结构图</h2><p>以上示例类的结构图如下所示</p>
<p><img src="https://oss.zhaohaihao.com/photo/design-patterns/lod/lod01.png?x-oss-process=style/style01" alt="image.png"></p>
<h1 id="问题所在"><a href="#问题所在" class="headerlink" title="问题所在"></a>问题所在</h1><p>前面提到过，迪米特法则强调只和密友交流。</p>
<p>在 <code>Boss</code> 类当中， <code>Staff</code> 作为方法的入参可以作为直接的朋友类。而 <code>Goods</code> 是出现在方法体内部的类，并不属于朋友类。</p>
<p><code>Boss</code> 直接要求 <code>Staff</code> 查询商品的总数量，只需要将结果告知即可。 <code>Boss</code> 并不需要关注 <code>Goods</code> 这个类。</p>
<h1 id="方案调整"><a href="#方案调整" class="headerlink" title="方案调整"></a>方案调整</h1><h2 id="调整老板类"><a href="#调整老板类" class="headerlink" title="调整老板类"></a>调整老板类</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Boss</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 老板要求职工查商品的数量</span></span><br><span class="line"><span class="comment">     * <span class="doctag">@param</span> staff 职工</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">requireCheckNumber</span><span class="params">(Staff staff)</span> </span>&#123;</span><br><span class="line">        staff.checkNumber();</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="调整职工类"><a href="#调整职工类" class="headerlink" title="调整职工类"></a>调整职工类</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Staff</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 职工查询商品数量</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">checkNumber</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        List&lt;Goods&gt; goodsList = <span class="keyword">new</span> ArrayList&lt;Goods&gt;();</span><br><span class="line"></span><br><span class="line">        <span class="comment">// 这里模拟从数据库查商品数量</span></span><br><span class="line">        <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i &lt; <span class="number">50</span>; i++) &#123;</span><br><span class="line">            goodsList.add(<span class="keyword">new</span> Goods());</span><br><span class="line">        &#125;</span><br><span class="line">        System.out.println(<span class="string">"目前超市内商品总数为: "</span> + goodsList.size());</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="类结构图-1"><a href="#类结构图-1" class="headerlink" title="类结构图"></a>类结构图</h2><p>调整后的整体类结构图如下</p>
<p><img src="https://oss.zhaohaihao.com/photo/design-patterns/lod/lod02.png?x-oss-process=style/style01" alt="image.png"></p>
<p>通过前后对比类图，我们不难发现，调整后的类结构中 <code>Boss</code> 类和 <code>Goods</code> 类没有直接的关联关系。</p>
<h1 id="优点"><a href="#优点" class="headerlink" title="优点"></a>优点</h1><ul>
<li>降低类之间的耦合，提高模块的相对独立性。</li>
<li>提高类的可复用性和系统的扩展性。</li>
</ul>
<h1 id="缺点"><a href="#缺点" class="headerlink" title="缺点"></a>缺点</h1><p>过度使用迪米特法则会使系统产生大量中介类，从而增加系统的复杂性，使模块之间的通信效率降低。</p>
<h1 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h1><p>迪米特法则主要强调以下两点：</p>
<ul>
<li>从依赖者的角度来说，只依赖应该依赖的对象。</li>
<li>从被依赖者的角度说，只暴露应该暴露的方法。</li>
</ul>
<h2 id="注意点"><a href="#注意点" class="headerlink" title="注意点"></a>注意点</h2><ul>
<li>在类的划分上，应创建弱耦合的类。类与类之间的耦合越弱，就越有利于实现可复用的目标。</li>
<li>在类的结构设计上，尽量降低类成员的访问权限。</li>
<li>在类的设计上，优先考虑将一个类设置成不变类。</li>
<li>在对其他类的引用上，将引用其他对象的次数降到最低。</li>
<li>不暴露类的属性成员，而应该提供相应的 get 和 set 方法。</li>
<li>谨慎使用序列化 (Serializable) 功能。</li>
</ul>
<h1 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h1><ul>
<li>《Head First 设计模式》</li>
<li>《大话设计模式》</li>
</ul>

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